The present invention relates to the transportation of powdered materials and more particularly to a method of delivering powdered coal, concentrate, or combustible powder to a burner of a furnace so that its burning may be more closely controlled.
In burning powdered coal or concentrate, the powdered material is usually entrained directly from a feeder (such as a screw feeder) into a high velocity air stream which enters the furnace at a burner. To ignite such entrained powdered coal or concentrate especially in a cold furnace the use of auxiliary gas or oil burners is required. A problem with this procedure is that it may be difficult to maintain ignition in the powdered coal burners since feed rates can be irregular or interrupted.
It is also difficult to ensure complete combustion of this material as it passes through the furnace. This creates problems because when coal is being burned it is necessary that the solid effluents from the furnace such as, for example, incompletely burned coal particles be kept to a minimum, not only from the standpoints of health or environmental considerations and of fuel economy, but also for minimizing fouling or carbonization of waste heat boilers.
Flame-out of burners is also a major concern. More particularly, when the feed of combustible materials to the burner is interrupted, the burner may go out or flame-out. Then, when feed is resumed, a large dose of powder will enter the burner and be ignited all at once. Such a flame-out may lead to dust explosions in the furnace. Burners used in the non-ferrous metals industry which are oxygen concentrate or so called "sprinkle" burners are particularly susceptible to flame-out. The "concentrates" fed to these burners are materials such as iron sulfide, copper sulfide and the like, hereinafter referred to simply as concentrate. This type of burner may be inserted in the roof of a reverbratory furnace. Control of the concentrate feed rate to the oxygen burner is central to its proper operation since interrupted feed rates can cause flame-out and hazardous dust explosions within the furnace when concentrate feed resumes suddenly. Such explosions may be damaging to both personnel and the furnace itself.
Numerous attempts have been made to feed fuel to burners in such a manner so as to alleviate the aforementioned problems. However, these feed methods have not satisfactorily overcome the problems involved.
If it is attempted to feed the powdered dense concentrates from a storage reservoir using a vibrated feed tube or hopper, packing tends to occur so that powder leaves the feed tube in clumps, falling through the burner without ignition and collecting in a pile beneath it.
At other times, concentrate fed from a vibrated hopper flows erratically into the oxygen burner, leading to interrupted ignition with the consequent danger of dust explosions.
Experimental fluidized beds have also previously been used to feed, for example, pulverized coal. Thus, Altenkirch et al., in their article entitled "Fluidized Bed Feed of Pulverized Coal", Powder Technology 20 (1978) 189-196, report the use of a standard fluidized bed with bottom distribution plate and a means to extract the fluidized coal from the bed. While this system performs well at low coal flow rates, air blockage of the off take tube occurs occasionally or frequently depending on off take design. Dilution air is also necessary when using an off take tube so as to prevent settling out of the coal in the tube and the resultant plugging of the tube. Unplugging the tube is time consuming and often results in changes in coal flow when feeding is resumed. In addition, large volumes of air are needed to fluidize the bed only a small fraction of which is extracted by the off take tube.
It would therefore be desirable to provide a method and device for fluidization of combustibles that would safely permit the feeding of large quantities of fluidized powdered fuel entrained in small quantities of gas directly to a burner.
Furthermore, a more controllable and reproducible method of coal combustion would be desirable, one that would ensure combustion of coal in such a manner that carbonaceous solids would persist to, but not beyond, the bounds of the working furnace chamber.
Additionally, in power plant boilers, it would be advantageous to burn coal powder containing sulphur in excess of that presently allowed without exceeding sulphur dioxide emission limits in the combustion off-gas.
Finally, it would also be desirable to able to burn low volatile coals or chars without danger of flame-out.